Mold. It's BAD NEWS. As well as just being gross it can do some pretty rotten things to your lungs. Prevention is definitely the way to go, as once you've got mold it's the very devil to get rid of. Luckily, you don't need harsh chemicals to stop mold arriving and bringing 50 gazillion of it's friends. Jennifer Chait, green-mama-maven over at Inhabitots has a recipe for an all-natural mold prevention spray.
You mix two cups of water and three drops of pure organic tea tree essential oil, pop it in a spray bottle and spritz (then wipe) your walls once a week. Simple!
Tea tree has lots of lovely antiseptic qualities, have a peek here for other things you can do with it.
Have a peek at the stacks of other homemade green cleaning supplies she has too.
The image is from here, probably best not let it get to that stage though...